\section{和差化积}
\begin{align*}
    \text{帅}+\text{帅}&=\text{帅哥，} & \sin \alpha + \sin \beta &= 2\sin \frac{\alpha + \beta}{2} \cos \frac{\alpha - \beta}{2} \\
    \text{帅}-\text{帅}&=\text{哥帅，} & \sin \alpha - \sin \beta &= 2\cos \frac{\alpha + \beta}{2} \sin \frac{\alpha - \beta}{2} \\
    \text{哥}+\text{哥}&=\text{哥哥，} & \cos \alpha + \cos \beta &= 2\cos \frac{\alpha + \beta}{2} \cos \frac{\alpha - \beta}{2} \\
    \text{哥}-\text{哥}&=\text{负嫂嫂。} & \cos \alpha - \cos \beta &= -2\sin \frac{\alpha + \beta}{2} \sin \frac{\alpha - \beta}{2}
\end{align*}